A Swiss meringue buttercream is a buttercream based on a sugar and egg white mix that is first heated and then whipped. The average recipes I could find use roughly one part egg white to two parts of sugar1, some a bit more, some less2.

I would like to reduce the sugar, but of course the sugar is not used just as sweetener, but is critical for the stability and texture of the meringue. So:

What’s the minimum amount of sugar or lowest sugar-to-egg ratio needed for a Swiss meringue buttercream?

Two side notes:

As egg sizes definitions differ and we’re talking ratios, please either use weight or state the egg size and the locale in your answer.

This question is not about the butter to meringue ratio. I realize that the amount of butter will influence the stability of the buttercream.

I doubt you can separate the effect of the butter (i.e. I suspect the answer would be a function of butter content, considering the limiting cases). I wouldn't mind being proved wrong though, and I think the question is interesting (+1).
– Chris HNov 6 '18 at 17:47